He went to Ulster to avenge the death of his nephew Cú Roí mac Dáire mac Dedad and wrought much destruction upon the land on his way, being invincible due to his tough skin "as of horn" deflecting spears and swords.
He sent his own daughter to him, Níab, to gain his trust, and offered him "feasts for a hundred men every afternoon."
Níab asked her new husband how he could be killed; Conganchnes replied that red-hot spits must be stuck into his soles and thrust up into his shins.
Níab relayed this information to her father, and told him to prepare a sleeping spell and a large army as well.
Celtchar then decapitated him; over his head rose a cairn, every man who passed it adding a stone.